The present invention relates to a glow plug used for preheating a subcombustion or combustion chamber of a diesel engine and, more particularly, to a self-temperature control diesel engine glow plug which performs fast heating, has a self-temperature saturation characteristic, and achieves after glow for a long period of time.
A diesel engine generally has a poor ignition characteristic at low temperatures. Therefore, a glow plug is attached to each subcombustion or combustion chamber. The glow plug is supplied with a current to increase an air intake temperature or serves as an ignition source so as to improve the ignition characteristic of the engine. Various types of conventional glow plugs are known. For example, the present applicant proposed a self-temperature control glow plug elaborately incorporating a resistor consisting of two types of materials, thereby obtaining stable heating characteristics so as to obtain a fast heating characteristic and prevent overheating of a heating element, as in Japanese Patent Laid-Open No. 57-182026.
In a glow plug of this type, a first resistor serving as a heating element and a second resistor connected in series therewith and consisting of a material having a positive resistance-temperature coefficient larger than that of the first resistor are embedded in a heat-resistant insulating powder in a sheath. A gap is formed between the first and second resistors to provide a time lag of heat conduction from the first resistor. A necessary high power is supplied to the first resistor upon its energization to quickly heat the first resistor, thereby assuring fast heating. At the same time, an increase in resistance of the second resistor upon an increase in temperature causes reduction of the power supplied to the first resistor after a lapse of a predetermined period of time. Melting of the first resistor which is caused by overheating can therefore be prevented, thus achieving the self-temperature saturation characteristic. With the glow plug having such a structure, a temperature control means or the like for controlling power supply to an energization circuit for a conventional glow plug must be arranged, and the total cost of the preheating unit becomes high.
In the conventional glow plug described above, although the fast heating function and the self-temperature saturation function can be assured to some extent, it is difficult to provide a heating characteristic for reducing a heating temperature in an after glow state upon starting of the engine. An after glow state lasts about several tens of seconds in the conventional glow plug. Strong demand has arisen for prolonging the after glow time over 10 minutes. No conventional glow plug can satisfy this demand. In order to perform the after glow operation for such a long period of time while the heating temperature is gradually reduced, a voltage drop resistor, a relay, and the like must be arranged in the energization circuit for the glow plug, and the circuit cost becomes high.
In order to allow the after glow for a long period of time by a glow plug itself without adding elements to the circuit, the energization power supplied to the heating element must be self-controlled to greatly improve the heating characteristics so as to prevent overheating of a heater portion. At the same time, a saturation temperature must be reduced to an appropriate temperature or less to assure durability of the heating wire and to maintain its temperature, thereby providing the self-temperature control function. In consideration of the above situation, demand has arisen for a glow plug having a heater portion having fast heating and self-temperature saturation functions and excellent reliability such as high heat resistance.
The present applicant proposed a glow plug structure to solve the above problem in Japanese Patent Laid-Open No. 60-117030. In this glow plug, second and third resistors made of a material having a positive resistance-temperature coefficient larger than that of a first resistor serving as a heating element are connected in series with the first resistor. The temperature rise of the third resistor is lagged from that of the second resistor, Therefore, the power supplied to the first resistor serving as the heating element in the after glow state is lower than that corresponding to the saturation temperature at the time of the start of the engine. In this structure, however, since a sheath in which the first, second, and third series-connected resistors are embedded in a heat-resistant insulating powder is straight, and the sheath has the same diameter along its longitudinal direction, a heat capacity of the second and third resistors serving as a control side is almost equal to that of the first resistor serving as the heating element. In addition, energization control is performed without considering the magnitudes of the positive resistance-temperature coefficients of the second and third resistors. Therefore, the increases in temperature of the second and third resistors do not cause a great time lag. This conventional glow plug is insufficient to obtain appropriate heating characteristics for reducing a heating temperature, assuring durability of resistors, and realizing the after glow for a long period of time.
Still another conventional glow plug is proposed in Japanese Utility Model Laid-Open No. 61-181957. In this glow plug, a second resistor having better heat-resistant, anti-corrosion properties than those of a third resistor and having a positive resistance-temperature coefficient is interposed between the third resistor located on the control side and a first resistor serving as a heating element. This glow plug solves a durability problem posed by the fact that the second resistor is excessively heated by a thermal influence of the first resistor and heat generated by the second resistor itself. This glow plug cannot achieve the after glow for a long period of time and cannot provide necessary heating characteristics. Some countermeasures must be taken to solve the problems described above in consideration of the above situation.